High-quality factor (high-Q) inductors improve the performance of RF front end components such as power amplifiers, filters, and diplexers. For example, high-Q inductors increase the efficiency and lower distortion and harmonics for power amplifiers. In addition, high-Q inductors enable lower insertion losses and higher out-of-band rejections in filters and diplexers. Although an RF design may thus require the use of inductors having a quality factor of 50 or greater (or even 70 and greater), the integration of high-Q inductors into a compact integrated circuit package faces a number of challenges. For example, the quality factor for an inductor is inversely related to its DC resistance (Rdc). An inductor having a relatively thick cross-section will have a lower DC resistance than a comparable inductor having a thinner cross-section. To better appreciate why a thicker inductor cross-section provides less DC resistance, consider the water flow produced by a fire hydrant as compared to a household faucet. They are both driven with substantially the same water pressure but the fire hydrant of course provides much greater flow. Similarly, a relative thick wire has less DC resistance than a relatively thin wire made of the same conductive material. It is thus trivial to achieve a high-Q inductor for an RF front end using a discrete conventional inductor for non-mobile applications in which space is not an issue because of the relatively thick conductors used to form such discrete inductors. But there is no space for such bulky discrete inductors in a compact integrated design. One approach is thus to integrate the inductor into the metal layers on the die. But the metal layer thickness in modern CMOS processes is too thin such that the DC resistance of the resulting embedded inductor is relatively high. As a result, the quality factor for die-embedded inductors is limited to approximately ten, which is too low for a high-fidelity RF design.
Embedded inductor designs have also been developed in which the inductor is formed in the metal layer (or layers) for the die package substrate. The quality factor and Rdc of such embedded inductors is dominated by the metal layer thickness as discussed above for die-embedded inductors such that the maximum quality factor is approximately 30 for a package-substrate-embedded inductor at 1.0 Giga-Hertz (GHz). In addition, even if the metal layer thickness could be increased, such thicker layers increase the tolerance needed for their etching. For example, rather than ending up with a desired square or rectangular inductor cross-section, an inductor formed from a relatively-thick etched metal layer may have a more trapezoidal-shaped cross section due to the nature of the metal etching process. One would thus have to design the inductor layout to compensate for the differences between a relatively thick base width and a narrower top width for the inductor cross section. The increased area needed to accommodate the etching error tolerance can thus lead to a need for completely redesigning the patterns for etching devices in a thicker metal layer.
Accordingly, there is a need in the art for improved high-Q embedded inductors for compact integrated circuit packages.